1. Field of the Invention
The present invention relates to an optical sheet having convex shape on the surface, a method for producing it, and a display apparatus including the optical sheet.
2. Description of the Related Art
In these years, cathode ray tubes (CRT), which were a mainstream of display apparatuses in the past, have been replaced by liquid crystal display apparatuses, since the liquid crystal display apparatuses have advantages such as low electrical power consumption, space-saving feature, and low cost.
There are several types of the liquid crystal display apparatuses when categorized by, for example, illumination methods in displaying images. As a representative example, a transmissive display apparatus which displays images by utilizing a light source arranged behind a liquid crystal panel can be cited.
In mobile products driven by a battery including such a display apparatus, the electrical power consumption of the display apparatus is enormously large, which is an obstacle to extend battery life. Specially, the ratio of the electrical power consumption of the backlight used in the display apparatuses is enormously large. Therefore, by reducing the electrical power consumption as much as possible, the battery life can be extended. In the result, practical values of the mobile products can be improved. However, it is not preferable that the electrical power consumption of the backlight is simply reduced, since luminance of the backlight is largely lowered, and thereby visibility of screen display of the display apparatus becomes unfavorable. Therefore, some measures to reduce the electrical power consumption of the backlight without largely lowering the luminance of the backlight have been proposed.
For example, in Japanese Unexamined Utility Model Application Publication No. 3-69184, a measure that an optical sheet having a plurality of pole prisms on the surface is arranged between a liquid crystal panel and a light source has been disclosed. A description will be specifically given of the optical sheet by using FIG. 12 and FIG. 13.
FIG. 12 shows an example of a cross sectional structure of a transmissive display apparatus 100 including lens films 112 and 113 as the foregoing optical sheet. FIG. 13 shows an enlarged view of part of a cross section of the lens film 112. The display apparatus 100 mainly includes a liquid crystal panel 120 and an illumination system 110 (so-called backlight) arranged behind the liquid crystal panel 120 (that is, opposite side of an observer side).
The illumination system 110 includes a light source 111, the lens films 112 and 113, a diffusion sheet 114, a lamp reflector 115, an light guide plate 116, and a reflecting sheet 117. The liquid crystal panel 120 mainly includes a polarizing plate 121, a transparent substrate 122, a color filter 123, a transparent electrode 124, an orientation film 125, a liquid crystal layer 126, an orientation film 127, a transparent pixel electrode 128, a transparent substrate 129, and a polarizing plate 130 in this order from the observer side.
In the display apparatus 100, light emitted from the light source 111 is reflected by the lamp reflector 115 and the reflecting sheet 117, directed in the direction of the liquid crystal panel 120, spread over the whole area of the liquid crystal panel 120 by the light guide plate 116, evenly diffused by the diffusion sheet 114, condensed by the lens films 112 and 113, and emitted to the liquid crystal panel 120. The light entering the liquid crystal panel 120 is transmitted to the observer side according to a size of a voltage applied to each pixel by a not-shown drive circuit.
As above, by arranging the lens films 112 and 113 between the liquid crystal panel 120 and the diffusion sheet 114, light emitted from the light source 111 can efficiently enter the liquid crystal panel 120. In the result, the electrical power consumption of the backlight can be reduced without largely lowering luminance of the backlight.
For example, as described in Japanese Unexamined Patent Application Publication Nos. 4-356746, 5-314545, 5-325272, and 6-47806, the lens films 112 and 113 can be produced by coating a transparent base with an uncured hardening resin, pressing a mother die having a reversal shape of pole prisms onto the hardening resin, and curing the hardening resin in such a state to transcribe the pole prism shape.